Veteran Republican U.S. Rep. Tim Johnson of Urbana, viewed as almost certain to win another term in November, will announce as soon as Thursday he is not seeking re-election, sources familiar with the Republican lawmaker’s decision said Wednesday.

The congressman, known for extensively phoning and traveling to visit with constituents, has grown unhappy with the job, according to one source long familiar with Johnson and Republican politics.

In addition, Johnson, who has a fitness regimen, became concerned about his own health after the recent stroke suffered by Republican U.S. Sen. Mark Kirk, the source said.

Another source said Johnson felt comfortable stepping down after David Gill, a Democrat from Bloomington, won the Democratic primary last month. Gill had run and lost against Johnson three times previously.

Johnson, a former state lawmaker first elected to Congress 12 years ago, has always marched to his own drummer.

He was the lone GOP member of Congress not to join an ultimately unsuccessful federal lawsuit challenging the Democratic redrawing of the state’s congressional districts. Johnson also was one of only a handful of members of Congress across the country to back Ron Paul’s bid for the GOP presidential nomination.

A list of potential replacements for Johnson on the November ballot rapidly developed.

Names being discussed include state Sen. Bill Bradyof Bloomington, the unsuccessful 2010 nominee for governor, state Rep. Chapin Rose of Mahomet, who won nomination to the state Senate last month, former state Sen. Duane Noland of Blue Mound, Johnson’s former chief of staff Jerry Clarke, and Rodney Davis, an aide to U.S. Rep. John Shimkus of Collinsville, who has worked on congressional get-out-the-vote efforts for the state GOP.